1. Technical Field
The invention relates to a printing technology of performing printing by relatively moving a print head with respect to a printing medium in a main scan direction and a sub scan direction.
2. Related Art
Recently, serial ink jet printers that perform printing by ejecting ink while relatively moving a print head with respect to a printing medium in a main scan direction and a sub scan direction have been provided. In general, in order to increase the printing speed, the serial ink jet printer employs bidirectional printing, in which ink is ejected in bidirectional (that is, forward-moving directional and backward-moving directional) main scans between the forward-moving main scan and the backward-moving main scan of the print head (refer to, for example, JP-A-2007-49443).
However, the bidirectional printing scheme has problems that are caused by mechanisms thereof. For example, if a printing apparatus reciprocatingly moves a print head from the left end of a printing medium in the main scan direction as one reciprocating movement, in the right end of the printing medium, just after dots are formed by the forward movement of the print head, dots are formed by the backward movement thereof. On the other hand, in the left end of the printing medium, just after the forward movement starts, dots are formed; and after a reciprocating time of the print head elapses, dots are formed by the backward movement thereof. Since the difference in dot forming timings is changed every time that the print head is moved in the sub scan direction, the difference causes non-uniformity in concentration in the sub scan direction in units of sub scan direction transporting amount (this phenomenon is also described in detail in the embodiments). This problem is particularly serious in a large-paper printer, which needs a relatively long time for the reciprocating movement of the print head.